The field of the invention is Group III-V heterojunction bipolar transistors (HBTs). The invention is particularly useful in transistors having a gallium arsenide (GaAs) or indium phosphide (InP) substrate.
For various performance reasons, some conventional HBTs use a collector layer with a wider bandgap than the bandgap of the base layer. The wide bandgap collector in an npn HBT blocks holes from entering the collector layer when the transistor saturates. This decreases saturation voltage of the transistor. The collector layer can harm performance, though, since the candidate materials used in the collector also tend to have a higher conduction band energy than that of the base layer. In that case, typical in HBTs having GaAs and InP substrates, the collector layer also partially blocks electron transport. This is evidenced by a small spike in the conduction band of the collector layer near the base-collector interface.
An improved HBT of the invention reduces the current blocking effect at the base-collector interface. Nitrogen is incorporated at the base-collector interface in an amount sufficient to reduce the conduction band energy of the collector at the base-collector interface to equal the conduction band energy of the base. In a preferred embodiment, a nitrogen concentration on the order of 2% is used in a thin xcx9c20 nm barrier layer at the base-collector interface. Preferred embodiment HBTs of the invention include both GaAs HBTs and InP transistors in various layer structures, e.g., single and double heterojunction bipolar transistors and blocked hole bipolar transistors.